The mature sperm morphology and spermatogenesis ultrastructure have been fully described in Pomatoleios krassii, which is a spirobranchian polychoaete attached to rocks, wrecked boats, rods and hard substrates at some polluted sites on Suez Bay coast. Sperms develop in the coelom as free-floating plasmodial cells. At the end of spermatogenesis, mature sperms float freely in the coelom. The spermatogonia are relatively large in size (13 μ in diameter). The spermatozoon of the present species differs from that of other polychaete species by having a large head (6 μ in length) formed of rounded nucleus and large acrosome which forms a large curved terminal cap on the nucleus. This may be an indication that fertilization occurs by some mechanisms that limit exposure of such fragile gametes to any possiple pollution. Meanwhile, female of P.kraussii protects its eggs with a jelly mass attached to the branchial crown, and the sperm is required to penetrate the matrix of egg mass. The large pair of mitochondria (2 μ in diameter) and presence of two pairs of proximal and distal centrioles which are not recorded so for in previous studies, as well as the long tail may be correlated with a characteristic movement of the sperm to reach female branchial crown, where the egg masses are found.